CT Extremity — Contrast-Enhanced Soft Tissue
The soft-tissue compartment in question with generous margins, including the draining nodal basin where infection or tumour is suspected.
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Suspected soft-tissue abscess, necrotising soft-tissue infection or septic collection where MRI is unavailable or too slow.
- Characterisation of a soft-tissue mass in a patient who cannot undergo MRI.
- Assessment of gas in the soft tissues alongside the extent of any collection.
Technique
- A systemic venous-phase acquisition at roughly 60-80 s shows abscess rim enhancement and the extent of inflammatory change.
- MRI remains the superior test for soft-tissue characterisation; this protocol is a substitute chosen for availability or contraindication, and that should be stated.
Where it goes wrong
- CT under-characterises soft-tissue masses relative to MRI, and a confident benign call is rarely possible.
- Arterial timing under-shows the enhancing wall of a collection.
- Streak from adjacent metalwork degrades exactly the soft-tissue plane being interrogated.
Clinical questions that reach this study
Contrast
Typically 70-100 mL of non-ionic iodinated contrast at around 2-3 mL/s.
Acquisition
- Reconstruction
- Thin axial reconstructions with soft-tissue and bone kernels and multiplanar reformats.
- Preparation
- Position the region of interest as close to the isocentre as possible and, where feasible, out of the contralateral limb — scanning both legs together doubles scatter across the joint you care about. Declare metalwork on the request so a metal-artefact protocol can be selected in advance.
Phases
Each phase is authored once and shared across every protocol that uses it, so the physiology below is the same wherever you meet it.
- Systemic venous (soft-tissue) phaseTypically ~45–90 s after the start of injection, with most published soft-tissue protocols sitting at ~60–80 s. Region-specific: reported neck delays span roughly 50–90 s, and routine contrast-enhanced chest work is conventionally ~60 s. Confirm locally.
Arteries and veins have equalised, and soft tissue outside the liver is at or near peak interstitial enhancement. The transferable principle is that away from the liver the purpose of a venous acquisition has nothing to do with portal delivery: it is that (a) every vessel is now uniformly dense, so a vessel stops being mistaken for a node or a mass, and (b) contrast has had time to leak into the expanded interstitium of inflamed or neoplastic tissue, which is what makes an abscess wall, a necrotic node, a mucosal tumour or a phlegmon declare itself against normal tissue. This is therefore the general-purpose soft-tissue phase for neck, chest, extremities and superficial structures. It is deliberately NOT called portal venous: the portal venous phase is a liver-timed acquisition that merely happens to fall in the same window, and borrowing its name for a neck or limb study imports a hepatic timing rationale that does not apply and hides the fact that the right delay is set by the target tissue. The errors are symmetrical — too early and arteries are far denser than nodes while an abscess rim has not yet enhanced; too late and everything equilibrates, collapsing the lesion-to-background difference the study depends on.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- Prior contrast reaction and elective premedication· nurse pre scan
- Intravenous access adequate for the planned injection· radiographer at scan
- Metformin and iodinated contrast· radiographer at scan
- Child-sized technique and contrast dose· radiographer at scan
- Kidney function and intravenous iodinated contrast· radiographer at scan